Acid-Base Equilibrium

acid

**ARRHENIUS DEFINITIONS:**

Substances that __ionize__ in water to produce hydrogen ions, **H+(aq)**.

base

**ARRHENIUS DEFINITIONS:**

Substances that __dissociate__ in water to produce hydroxide ions, **OH-(aq)**.

acid

**BRONSTED-LOWRY DEFINITIONS:**

The proton, H+(aq), **donor** in a reaction.

* e.g. hydrofluoric acid + water

base

**BRONSTED-LOWRY DEFINITIONS:**

The proton, H+(aq), **acceptor** in a reaction.

* ammonia + water

amphoteric substance

Substances that can act as either an acid or a base.

* Water acts as a **base** with hydrofluoric acid, but as an **acid** with ammonia.

strong acid

Completely ionizes in water.

weak acid

Only a fraction of the molecules ionize in water.

conjugate acid-base pair

Molecules or ions related by the **transfer of a proton**. An acid becomes a conjugate base when a proton is removed, and a base becomes a conjugate acid when a proton added.

ion-product constant

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percent ionization

The **ratio** of the concentration of ionized molecules at equilibrium, to the initial concentration of the weak acid, expressed as a percent:

* A **strong acid** almost completely ionizes in water to form hydrogen ions, and its **% ionization > 99%**.
* A **weak acid** partially ionizes in water to form hydrogen ions, and most have a **% ionization < 50%**.

acid ionization constant

**Ka**

A **weak acid** partially ionizes, donating protons in water. At **equilibrium**, there will be **BOTH** the **weak acid**, HA, and its **conjugate base**, A-, in the solution.

monoprotic acid

Has **one** ionizable hydrogen atom.

* e.g. HCl (aq), HNO3 (aq), HClO (aq)

polyprotic acid

Has **more than one** ionizable hydrogen atom.

* e.g. H2SO4 (aq), H3PO4 (aq)

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With the __exception of sulfuric acid__, all of these acids are **weak**. They __*do not ionize completely in one step*__, but rather __*in two or more steps*__. Each step has an ionization constant: Ka1, Ka2, Ka3, etc.

base ionization constant

**Kb**

A **weak base** partially ionizes, accepting protons in water. There are two main categories:


1. **Neutral substances** with an atom that has a nonbonding pair of electrons that can accept H+ (e.g. ammonia and the amines).
2. **Anions** of weak acids (conjugate base).

At **equilibrium**, there will be **BOTH** the **weak base**, B, and its **conjugate acid**, BH+, in solution.

weak base

Partially ionizes, accepting protons in water.

salt

An ionic compound that forms during a **neutralization** reaction between an **acid** and a **base**. The **cation** (positive ion) comes from the **base**, and the **anion** (negative ion) comes from the **acid**.

hydrolysis

**A reaction in which one of the ions from a salt reacts with water, forming either an acidic or basic solution.**

Different salts will produce __various pH ranges__ in an aqueous solution depending on how the ions of the salt react with water to produce either H3O+ or OH- ions.

We can use the strengths of the “parent” acid and base to help us predict the pH of the salt solution.

We need to look at the **salt** and ask:


1. Which “parent” acid and base **reacted** to form the salt?
2. If the “parent” **acid/base** strong or weak?

* __***If it is a strong acid/base:**__ ***fully ionize in water, salt ion will NOT hydrolyze with water (acts as pH-neutral)***
* __***If it is a weak acid/base:**__ ***partially ionize in water, salt ion WILL hydrolyze with water (acts as weak base/acid)***

titration

Used to determine the **molar mass**, **concentration**, or **pH** of one of the compounds in a neutralization reaction.

equivalence point

Point in a titration when the acid and base that are present **completely react** with each other (**n(a) = n(b)**).

* Located in the middle of steep rise of titration curve.

endpoint

Point in titration where indicator **changes colour**.

* **pH = pKa** of the indicator (logKa)
* Pick an indicator that changes colour close to the equivalence point so there is **negligible volume** difference.